The invention relates to the field of communications systems and, more specifically, to apparatus and methods for xe2x80x9ctrainingxe2x80x9d an optical switch fabric.
The xe2x80x9ctrainingxe2x80x9d of an optical switch fabric is the process of adapting various parameters within the switch fabric such that the amount of power attenuation imparted to a switched optical signal is minimized. Training an optical switch fabric becomes increasingly time-consuming as the switch fabric density increases. For example, a 256xc3x97256 port switch fabric typically comprises 64 K connections, while a 1296xc3x971296 port switch fabric contains approximately 25 times more connections (i.e., 1.6 M connections).
A first method of training a switch fabric comprises using of a single optical input source that is used to sequentially apply an optical signal to each input port of the switch fabric. The switch fabric then sequentially couples the applied optical signal to each of the output ports. A single power monitoring device measures the output power of the optical signal provided at each output port. Various parameters within the switching fabric are adapted until the amount of power measured at the output port is maximized (or at least above a minimum threshold level). Advantageously, the use of a single power monitoring device such as a standard power meter allows for a level of sensitivity to power measurement fluctuations of approximately xe2x88x9280 to xe2x88x9290 dBm range, thereby providing great sensitivity such that parameter adjustments within a switch fabric may be accurately evaluated. Unfortunately, the speed at which an optical switch can be trained using this method depends on the speed with which the shared resources (i.e., the optical input source and the monitoring device) can be switched among the various input and output ports.
A second method of training comprises providing a dedicated optical input source for each of the input ports and a dedicated power monitoring device for each of the individual output ports. This method avoids the speed constraints placed upon the method described above due to the sharing of resources. A practicable system utilizing this technique requires power monitors such as p-i-n diodes, PN diodes or avalanche diodes at every input port and output port. While this technique allows for relatively quick training of the switch fabric, the limited dynamic range of the output port power monitors limits the accuracy of such training. Also, using these types of power monitoring devices, the level of sensitivity to power measurement fluctuations is reduced to approximately xe2x88x9235 dBm. Thus, relatively minor adjustments of switch fabric parameters may not produce enough fluctuation in output power measurement to be detected and, therefore, evaluated properly. In addition, a large amount of hardware is needed to drive and monitor all the ports on the switch to be trained.
The invention comprises a method and apparatus for training an optical cross-connect or switch fabric using a relatively small number of optical sources and relatively inexpensive power measurement devices. Specifically, energy provided by a single optical source is split into a plurality of reduced-power optical signals, each of the reduced power optical signals being coupled to a respective input port of a switch fabric. A controller causes each input port of the switch fabric to be sequentially coupled to each of a plurality of output ports where a respective power monitoring device is used to determine optical power losses within each optical path within the switch fabric. The controller adapts operational parameters of the switch fabric in a manner tending to reduce optical attenuation imparted to optical signals switched therethrough.